The present invention relates to hollow or tubular window frames of extruded plastics material and of the general type disclosed in U.S. Pat. No. 4,941,288 and U.S. Pat. No. 5,003,747 which issued to the Assignee of the present invention. In such a window frame, a pair of vertical jamb members are rigidly connected by a horizontal sill member and a horizontal head member, and each of the tubular members are formed by extruding a rigid plastics material such as polyvinylchloride (PVC). Preferably, the window frame is formed by mitering the opposite ends of the extruded linear frame members and then welding the mating mitered corners together to provide substantial rigidity.
The PVC frame may support a fixed picture window unit or one or more operable or movable window units, as shown in the above-mentioned patents. The frame is installed within a rough opening of a building wall, and sometimes the wall has an exterior surface covered by aluminum or PVC siding, usually in the form of lapped siding. After a window frame is positioned within the rough opening of a building wall, it is secured by nails or screws extending through a laterally outwardly projecting nailing flange extruded as an integral part of each frame member. When exterior siding is to be installed, it is common to install a separate J-channel around the exterior projecting portion of the window frame. The J-channel has a nailing flange portion which is attached to the building wall, a portion which covers the ends of the siding panels and a returned flange portion which overlies the end portions or edge portions of the siding panels.
A typical J-channel is disclosed in U.S. Pat. No. 4,608,800 and allows for longitudinal expansion and contraction of each siding panel while covering the expansion gap between the ends of the siding and the window frame. In factory manufactured housing units, it is known to construct a window frame with relatively flat extruded linear frame members and with an integrally extruded J-channel formed in part by the window frame nailing flange, for example, as disclosed in U.S. Pat. No. 5,660,010. A similar window frame construction is disclosed in U.S. Pat. No. 2,983,001. While the frame constructions disclosed in these patents may be acceptable for manufactured housing units, the frame structure would not be acceptable in site constructed homes where the window frame units must provide substantial strength, rigidity and an attractive appearance. It is also sometimes desireable for a window frame to provide for conveniently installing wood sill, head and jamb extenders within the interior of the building in order to provide a wood interior window frame for the window assembly or unit.